[Pw_forum] constraining Cartesian lattice vector components

[Paolo Giannozzi]

Have you looked at option "cell_dofree"?

P.

On Sun, 2014-03-16 at 01:41 +0000, Nemeth, Karoly wrote:
> Dear Quantum Espresso developers and users,
> 
> I was wondering whether it is possible to constrain individual Cartesian components of the lattice vectors during geometry optimization (vc-relax), at least for the ibrav=0 case. This would in principle be similar to constraining Cartesian atomic positions, however, it appears that this feature is not available in Quantum Espresso yet.
> 
> Also, it appears that during vc-relax the cell is slightly rotated. I would expect that the first two lattice vectors remain along the x axis and in the xy plane while the third one may have non-zero x, y and z components. In other words, if I start the optimization with the upper triangle of the cell_parameters matrix being all zero, I would expect that it stays so, and only the diagonals and the lower triangle will vary, which includes 6 numbers only. These six non-zero numbers represent the 6 lattice parameters in Cartesian space. Allowing all 9 numbers to vary in the cell_parameters matrix introduces redundancy that may lead to the rotation of the cell and slows down convergence.
> 
> I would greatly appreciate any advice in the above two (related) matters.
> 
> Thanks:
> 
> Karoly
> http://science.iit.edu/people/faculty/karoly-nemeth
> 
> 
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-- 
Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
Phone +39-0432-558216, fax +39-0432-558222